Aromatic hydrocarbon compounds such as benzene are frequently used for producing transportation fuels and petrochemicals such as styrene, phenol, nylon and polyurethanes and many others. Benzene is typically produced in processes such a steam cracking and catalytic reforming. During steam cracking, a C2+ hydrocarbon feed is exposed to high-temperature pyrolysis conditions to produce a product comprising molecular hydrogen, C4− olefin, other C4− hydrocarbon, and C5+ hydrocarbon. The yield of aromatic hydrocarbon from steam cracking is generally much less than the yield of light hydrocarbon. Consequently, complex processes typically are needed for separating and recovering aromatic hydrocarbon from steam cracker effluent. Catalytic naphtha reforming produces a product having a much greater content of aromatic hydrocarbon than steam cracker effluent, but the naphtha feed is itself useful for other purposes such as a motor gasoline blendstock.
Various attempts have been made to overcome these difficulties, and provide an efficient process for producing aromatic hydrocarbon at high yield from a relatively inexpensive feed. For example, processes have been developed for producing light aromatic hydrocarbon (e.g., benzene, toluene, and xylenes—“BTX”) from paraffinic C4− feeds. The processes typically utilize an acidic molecular sieve such as ZSM-5 and at least one metal having dehydrogenation functionality, such as one or more of Pt, Ga, Zn, and Mo. These conventional processes typically operate at high temperature and low pressure. Although these conditions are desirable for producing aromatic hydrocarbon, they also lead to undue catalyst deactivation as a result of increased catalyst coking. Catalyst coking generally worsens under conditions which increase feed conversion, leading to additional operating difficulties.
One way to lessen the amount of catalyst coking is disclosed in U.S. Pat. No. 5,026,937. The reference discloses removing C2+ hydrocarbon from the feed in order to increase the feed's methane concentration. Since ethane, propane, and butanes are less refractory, removing these compounds from the feed decreases the amount of over-cracking, and lessens the accumulation of catalyst coke. The process utilizes a catalyst comprising molecular sieve, an amorphous phosphorous-modified alumina, and at least one dehydrogenation metal selected from Ga, Pt, Rh, Ru, and Ir. The catalyst contains ≤0.1 wt. % of Ni, Fe, Co, Group VIb metals, and Group VIIb metals. The reference also discloses increasing aromatic hydrocarbon yield by removing hydrogen from the reaction, e.g., by combusting the hydrogen with oxygen in the presence of an oxidation catalyst that has greater selectivity for hydrogen combustion over methane combustion.
Processes have also been developed for converting less-refractory paraffinic hydrocarbon to aromatic hydrocarbon with decreased selectivity for catalyst coke. For example, U.S. Pat. No. 4,855,522 discloses converting C2, C3, and C4 hydrocarbon with increased selectivity for aromatic hydrocarbon and decreased selectivity for catalyst coke. The process utilizes a dehydrocyclization catalyst comprising (a) an aluminosilicate having a silica:alumina molar ratio of at least 5 and (b) a compound of (i) Ga and (ii) at least one rare earth metal. The reference discloses carrying out the aromatization conversion at a space velocity (LHSV) in the range of from 0.5 to 8 hr−1, a temperature ≥450° C. (e.g., 475° C. to 650° C.), a pressure of from 1 bar to 20 bar, and a feed contact time of 1 to 50 seconds.
More recently, Catalysts have been developed to further reduce the amount of catalyst coking during the dehydrocyclization of C4− paraffinic hydrocarbon. For example, increasing the catalyst's dehydrogenation metal loading has been observed to lessen the amount of catalyst coking. See, e.g., U.S. Pat. No. 7,186,871. But increasing dehydrogenation metal loading has been found to increase the catalyst's hydrogenolysis activity, resulting in an increase in the amount of methane and other light saturated hydrocarbon in the reaction product and a decrease in the amount of the desired aromatic hydrocarbon. This effect can be mitigated by further increasing catalyst complexity, e.g., by adding an attenuating metal to the catalyst as disclosed in U.S. Pat. No. 8,692,043.
Hydrogenolysis side-reactions can also be mitigated by carrying out the aromatization in two stages. For example, U.S. Pat. No. 8,835,706 discloses aromatization of an ethane-propane feed. The feed is obtained from natural gas by cryogenically separating methane. The feed is reacted in a first stage operated under conditions which maximize the conversion of propane to aromatics. Following separation of the aromatic hydrocarbon, ethane and any other non-aromatic hydrocarbon produced in the first stage are converted to aromatics in a second stage. The second stage is operated under conditions which maximize the conversion of ethane to aromatic hydrocarbon. Two fixed-bed reactors can be used in each stage. The process can be operated continuously by cycling between the first and second reactor in each stage, with the first reactor carrying out aromatization while the second reactor undergoes decoking, and vice versa. The patent discloses that increased catalyst coking can be overcome by utilizing fluidized catalyst beds in the reaction stages. Decreasing the amount of time (the “cycle time”) that a fixed bed reactor is operated in aromatization mode before switching to decoking mode can also be used to lessen the amount of coke accumulation.
Improved processes are needed for dehydrocyclization of light paraffinic hydrocarbon that exhibit one or more of a greater feed conversion, a greater yield of aromatic hydrocarbon, and a lesser yield of undesired byproducts such as catalyst coke and C4− hydrocarbon. Processes are particularly desired which can be carried out with catalysts of lesser complexity, in fixed catalyst beds with increased cycle time, and/or without the need for cryogenic separation.